The present invention relates generally to semiconductor device packaging and, in particular, to a structure and method for removably attaching a heat sink to an electronic package which is, even more particularly, a surface mount package.
In order to safely operate modem computers and other electronic devices, thermal energy must be removed from components which generate large amounts of heat and which cannot dissipate the heat fast enough to avoid harmful consequences. As the art moves towards smaller, higher-power integrated circuits such as static random access memory (SRAM) integrated circuits, heat transfer from the integrated circuit package (IC package) becomes increasingly difficult and more important. (The term xe2x80x9cIC packagexe2x80x9d includes the heat-generating integrated circuit as well as the packaging surrounding the integrated circuit.) As chip density and the speed of IC chips increase, chips often require high-performance surface mount packages assembled very close to each other on circuit cards. These packages generate heat and, because they are packed very close to each other, they become hot.
One conventional method used to dissipate the accumulating heat of an electronic device such as an IC package is to force air over the device, thereby removing the heat by convection. This method has been substantially improved by attaching a heat dissipating device, also known as a heat sink, to the electronic device. The heat sink is designed to dissipate heat at a significantly greater rate than the electronic device alone. A heat sink typically has projections such as pins or fins, exposing a greater surface area to the flow of air for greater dissipation of heat.
When the heat sink is placed in thermal contact with the electronic device, the electronic device transfers heat by conduction to the heat sink. The heat to sink then dissipates accumulating heat by convection to the ambient environment. This method has become standard in the computer industry for dissipating the large amounts of heat generated by the new generation of computer processors.
Of importance, the heat sink must be reliably attached to the electronic device. In the case of IC packages, such attachment must not undesirably is stress or damage the IC package or the circuit board to which the IC package is connected. One conventional technique is to employ a thermally conductive adhesive which bonds the heat sink to the IC package. Thermally conductive adhesives do not adhere well to plastic IC packages, however, resulting in an unacceptably high incidence of bond failure between the plastic IC package and the heat sink. Further, once the heat sink is bonded with the thermally conductive adhesive, it is difficult to remove the heat sink from the IC package without causing damage to the IC package, the heat sink, or the circuit board. Still further, it is desirable to have a removable heat sink to readily allow chip repair, rework, or replacement. Accordingly, it is preferable to avoid the use of thermally conductive adhesives altogether.
Other methods have been used to secure the heat sink to the electronic device, including the use of clips to fasten the assembly together. It is preferred that any attachment method allow for quick and easy installation and removal of the heat sink while providing a secure attachment during operation and normal handling. Electronic devices must be removed and replaced from time to time; therefore, the heat sink must be easy to remove and install without sacrificing reliability or effectiveness. Clips offer the advantages of being relatively inexpensive, simple, operational over a multiple number of assembles and disassembles, and reasonably secure so that the heat sink does not disconnect or dislodge from the electronic device.
A wide variety of different clips are known. Two-piece dips are disclosed by U.S. Pat. No. 5,617,292 issued to Steiner. As Steiner teaches, however, previous two-piece clips used to fasten heat sinks generally lack sufficient rigidity and strength, particularly at the junction between the two parts of the clip, and separation of the assembly is a potential risk. In general, two-piece clips are constructed so that the intersection of the separate leg and the leaf spring member forms a loose hinge about which rotation at least to some degree in several directions is possible. Thus, the force holding the assembly together is exerted substantially entirely in a direction normal to the electronic component. The hinge mechanism is relatively weak and prone to failure, particularly under prolonged use and shock and vibration. Holding the necessary tolerances during manufacturing imposes additional difficulty in producing two-piece clips.
One-piece clips avoid some of the drawbacks inherent in two-piece clips. Such clips generally extend over the heat sink and attach at each end to the electronic device or its socket at specially provided ports or bosses. Clips of this design require modification of the electronic device or its socket and may deleteriously affect performance of those components. To avoid these problems, other clips attach removable heat sinks directly to the circuit board to which the IC package is connected.
For example, Tseng discloses, in U.S. Pat. No. 6,043,984 assigned to Intel Corporation, a removable heat sink that uses clips or fasteners to attach the heat sink to an IC package. The IC package is mounted to a substrate such as a circuit board. The clip has four L-shaped ears that are inserted through clearance holes in the substrate and corresponding attachment holes in the heat sink. The four L-shaped ears extend from a center plate portion. The L-shaped ears are bent during insertion through the holes. Once beyond the attachment holes of the heat sink, the L-shaped ears snap back so as to permit the clip to exert a spring force that pushes the heat sink into the integrated circuit package.
As described in U.S. Pat. No. 6,154,365, also assigned to Intel Corporation, the clip taught by Tseng has several drawbacks. From one electrical assembly to the next, the spring force from the L-shaped ears is not consistent due to the variations within part manufacturing tolerances. Moreover, the L-shaped ear technique does not permit a technician to adjust the spring force from the L-shaped ears so as to evenly distribute this force over the surface of the substrate. Without an even distribution of force, the substrate is more likely to bend which may cause the substrate or the integrated circuit within the integrated circuit package to crack. When attached directly to the IC package in the manner disclosed, the heat sink exerts undue force on the IC package which can damage and ultimately destroy the IC package.
Post-type fastening members are also used to secure a heat sink to an electronic device. FIG. 1 is a side view of an electronic device 8 which includes a heat sink 10 directly attached by post-type fastening members 12A to a circuit board 14. Located between the heat sink 10 and the circuit board 14 is an IC package 16 which generates heat during use. The IC package 16 is typically electrically connected to the circuit board 14 by one or more circuit interconnections, e.g., solder, which are not illustrated in FIG. 1 for purposes of clarity. The fastening members 12A urge the heat sink 10 towards the circuit board 14 and down onto the IC package 16 to make the thermal contact between the heat sink 10 and the IC package 16.
Although providing the force necessary to make the thermal contact between the heat sink 10 and the IC package 16, the fastening members 12A cause the heat sink 10 to press unevenly on the IC package 16. In particular, the IC package 16 acts as a pivot between the heat sink 10 and the circuit board 14 so that the end 10A of the heat sink 10 is urged away from the end 14A of the circuit board 14 as indicated by the arrows 18. This causes the force exerted by the heat sink 10 on the IC package 16 to be greater at the side 16A of the IC package 16 than at the side 16B. This uneven force distribution can damage and even crack the IC package 16. Further, this uneven force distribution can create a gap between the side 16B and the heat sink 10 resulting in poor heat transfer between the IC package 16 and the heat sink 10. Alternatively, or in addition, this uneven force distribution can cause circuit interconnection failure near the side 16B of the IC package 16. As those skilled in the art understand, these conditions can ultimately cause failure of the device 8.
To avoid these drawbacks, it has become known in the art to attach both sides of the heat sink 10 to the circuit board 14. As an example, second post-type fastening members 12B illustrated by dashed lines in FIG. 1 can be employed. This tends to equalize the force exerted by the heat sink 10 on both the sides 16A and 168 of the IC package 16. This also causes the ends 14A, 14B of the circuit board 14 to be pulled up, however, by the fastening members 12B, 12A, respectively, relative to the die attach region 14C of the circuit board 14 to which the IC package 16 is attached. This bending force, indicated by arrows 20, causes the circuit board 14 to warp such that the circuit board 14 is displaced to a position 22. Over time, such bending can cause the device 8 to fail, e.g., from failure of circuit interconnections between the IC package 16 and the circuit board 14. U.S. Pat. No. 6,125,037, issued to Bollesen, discusses the post-type fastening members illustrated in FIG. 1 and described above.
FIG. 2 shows a product designed by Intel Corporation illustrating still another mechanism used to secure a heat sink to an electronic device. As with many electronic packages used in the computer industry, the Intel product includes a layered region of five stacked components: (1) a circuit board 14, (2) a socket 30, (3) a processor (not shown), (4) a thermal pad (also not shown), and (5) a heat sink 10 designed to dissipate heat generated by the processor. The heat sink 10 is retained in position over the processor, for the illustrated structure available from Intel, by a pair of retention modules 40 placed alongside two opposing sides of the heat sink 10. Two screws 42 engage mating screw holders 44 in the circuit board 14 to affix each retention module 40 in position. A spring clip 46 fits over each retention module 40 and exerts a downward force on an edge portion of the heat sink 10, peripheral to the cooling fins of the heat sink 10, to retain the heat sink 10.
Thus, the conventional Intel structure requires multiple components. The Intel structure specifically requires two retention modules 40, four screws 42, four screw holders 44, and two spring clips 46. The spring clips 46 require a separate tool for assembly and disassembly. When the first spring clip 46 is assembled, the downward force exerted on an edge of the heat sink 10 pivots upward the unrestrained opposite side of the heat sink 10. Such pivoting action may damage the interface (typically a thermally conductive pad, grease, or oil) between the heat sink 10 and the processor.
To overcome the shortcomings of conventional mechanisms, a new device and method for retaining heat sinks on electronic devices such as IC packages is provided. An overall object of the present invention is to provide an improved heat sink retention structure and method. Another object is to reduce the space occupied by the retention structure on the underlying electronic package.
Still another object is to ease the assembly and disassembly of the heat sink. A related object is to eliminate some of the components required by conventional retention structures, including separate tools for assembly and disassembly. Additional objects of the present invention are to locate the retention force substantially centrally on the heat sink or in the region of the fins of the heat sink and to distribute the force evenly. It is still another object to avoid possible damage to the package or thermal interface material during assembly and disassembly of the heat sink. Yet another object of this invention is to provide stiffness to and avoid bending of the circuit board which underlies the heat sink. Also an object is to minimize the disruption of air flow through the heat sink.
To achieve these and other objects, and in view of its purposes, the present invention provides a structure for removing heat from a packaged electronic component. The structure includes three, main elements. The first element is a heat sink having a pedestal with opposing ends and a plurality of fins separated by spaces and disposed on the pedestal. The heat sink dissipates heat generated by the packaged electronic component. The second element is a set of parallel rails disposed adjacent the opposing ends of the pedestal of the heat sink, each rail having a catch. The third element is a spring clip having (a) end hooks which engage the catches on the rails to retain the spring dip on the rails, and (b) a strut extending between the hooks, fitting into the space between adjacent fins of the heat sink, and including an apex which contacts substantially centrally the pedestal of the heat sink and applies a force pressing the heat sink toward the packaged electronic component. Thus, the heat sink is removably attached at least indirectly to the packaged electronic component.
The present invention further provides an electronic package. The electronic package includes a circuit board; a socket affixed to the circuit board; a processor fitted in the socket and generating heat; a thermal pad engaging the processor and transmitting the heat generated by the processor; a heat sink having a pedestal with opposing ends and a plurality of fins separated by spaces and disposed on the pedestal, the heat sink engaging the thermal pad and dissipating the heat generated by the processor and transmitted by the thermal pad; and an attachment structure removably attaching the heat sink to the thermal pad. The attachment structure includes: (a) parallel rails disposed adjacent the opposing ends of the pedestal of the heat sink, each rail having a catch, and (b) a spring clip having (1) end hooks which engage the catches on the rails to retain the spring clip on the rails and (ii) a strut extending between the hooks, fitting into the space between adjacent fins of the heat sink, and including an apex which contacts substantially centrally the pedestal of the heat sink and applies a force pressing the heat sink toward the thermal pad.
The present invention still further provides a method of removably attaching a heat sink to a packaged electronic component whereby the heat sink dissipates heat generated by the packaged electronic component. The heat sink has a pedestal with opposing ends and a plurality of fins separated by spaces and disposed on the pedestal. The method includes arranging parallel rails adjacent the opposing ends of the pedestal of the heat sink, each rail having a catch. A spring clip is installed pursuant to the method. The spring clip has (a) end hooks which engage the catches on the rails to retain the spring clip on the rails, and (b) a strut extending between the hooks, fitting into the space between adjacent fins of the heat sink, and including an apex which contacts substantially centrally the pedestal of the heat sink and applies a force pressing the heat sink toward the packaged electronic component. Thus, the heat sink is removably attached at least indirectly to the packaged electronic component.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive, of the invention.